Process for chromium electrodeposition



March 15, 1955 TADASHI YOSHIDA 2,704,273

PROCESS FOR CHROMIUM ELECTRODEPOSITION I Filed liov. 10, 1951 40 5'0 7///. 51/4/ 5 4/75/5 PFE APAT/FA Haw- #9 Jim/19mm yum/5 IN VENTOR.

FDASHI YOSHIDA AGENTS.

United States Patent PROCESS FOR CHROMIUM ELECTRODEPOSITION Tadashi Yoshida, Tokorozawa City, Japan Application November 10, 1951, Serial No. 255,857

Claims priority, application Japan September 28, 1951 6 Claims. (Cl. 204-51) The present invention relates to a process for the electrodeposition of metallic chromium, wherein an aqueous solution containing chromic sulfate and free urea, with complex ions having reached an equilibrium at the temperature of electrolysis, is used as an electrolytic bath and at the same time a lead alloy containing relatively small quantities of tin, silver and cobalt is used as the anode.

As is well known, the chromic acid process industrially used at the present time involves considerable disadvantages, such as an abundant evolution of poisonous gases during operation, an exceedingly low current efiiciency, etc. Heretofore, a number of proposals have been made in regard to the use of a trivalent chromium bath which avoids the above-mentioned defects. So far, however, it seems that no process using a trivalent chromium bath has been reported, which has proved successful in industrial practice, excepting U. S. Patent No. 2,507,475 granted to Rex R. Lloyd on May 9, 1950, regarding the electrowinning where a diaphragm and lead anode are used for electrolysis.

The primary object of the present invention lies in establishing industrially valuable process for the electrodeposition of high quality metallic chromium at high efiiciency and without the evolution of poisonous gas during the operation using a chromic sulphate bath. An additional object of the invention is to obtain the further industrial advantages as follows:

1. An increased working efficiency based on sanitary working conditions; low consumption of chromium chemicals and electric power; no installation cost and upkeep expense for the exhaust equipment for poisonous gas;

2. Less tendency shown by the electrolytic bath of making the metal surface passive, when current is stopped during operation. This facilitates rotary basket plating for small articles.

The advantages of the present invention are set forth as follows: In the present invention, it is clear that the chromic sulfate bath should be kept at a constant electrolytic temperature for a relatively long interval of time, so that the complex ions in the bath may be given time enough to reach an equilibrium for the temperature of electrolysis, before it is used for the plating operation. According to the present inventors view, one of the principal reasons, why the usual processes for electroplating using chromic sulfate bath has not brought about thus far any success, can be said as follows: The former workers conducted merely a macroscopical observation, on the state of complex ions in the aqueous solution of chromic sulfate, for instance, if it is purple or green. They could not find out any reproducible working conditions of any industrial value, because they conducted their experiments of electrodeposition, with an electrolytic bath in which the complex ions were still in a transitional stage, without paying any attention to the intricated behaviors of said aqueous solution as described below.

Also in the aqueous solution containing chromic sulfate alone, in a majority of cases, two or more kinds of complex ions, whose principal metallic component is trivalent chromium, are coexistent. In addition, such state of complex ions as described above varies gradually, according to the given working conditions. Moreover, for each working condition given according to the inventors study, the state of complex ions in an aqueous solution of the same composition containing chromic sulfate, reaches a definite equilibrium at a given temperature after 'ice a certain time interval, despite of the initial state of those complex ions. In this case, an aging for a relatively long period of time is necessary, before an equilibrium of the state of complex ions has been reached in the solution, at a given temperature. It has been found that, only when an electrolytic bath, of which complex ions have reached to an equilibrium condition at the temperature of electrolysis, is subjected to an electrolysis, not only reproducible results are obtainable, but also an exceedingly satisfactory electrodeposition is obtainable, as compared with the case when an unstable complex ion electrolytic bath is used. A part of the inventors experimental study is shown on the accompanying drawing comprising a graph as follows: Three aqueous solutions (A, B and C) were prepared, containing each 0.5 mol of chromic sulfate and 3.2 mols of ammonium sulfate in a liter, while differing from one another in the following points:

(A) a solution containing 4.0 mols of urea besides the above substances in a liter, was subjected to boiling just before the addition of urea and the urea was dissolved therein after the solution was cooled down to room temperature; (B) the solution had a similar composition as the solution (A) but was kept at room temperature for about seven months after preparation; (C) an aqueous solution with no urea was sub ected to boiling. These three aqueous solutions were put in a thermostat adjusted at 40 (3., directly after they were prepared. With respect to each individual solution, the variation of conductivity was pursued. A diagram as shown in the figure has been obtained by plotting the time elapsed after the preparation on the abscissa and the specific conductivity of each solution on the ordinate axes respectively. The time needed for curve (A) and curve (B), in the figure, converting in straight lines parallel to the abscissa is dilferent. The two curves, however, are converted finally in one straight line which is parallel to the abscissa. The fact mentioned above proves that a state of complex ion, in a chromic sulfate bath of the same chemical composition, will reach a definite equilibrium at a given temperature in a certain period of time, despite of the initial state of the complex ion. Thus, both curve (A) and curve (C) become parallel to the abscissa in about 45 hours after they have been prepared. Namely, it has been shown that aging for about 45 hours is necessary for the state of complex ion in the electrolytic bath of above-mentioned composition, which has once been boiled, to come to an equilibrium at about 40 C. The optimum temperature for the electrolytic bath is found, according to the present invention, at a temperature between 25 and 55 C. At this temperature, the time for aging, needed by the complex ions of an electrolytic bath, which has once been subjected to boiling during its preparation, for attaining an equilibrium at the given temperature, can be obtained by a similar method as adopted in the above mentioned experiment.

The process according to the present invention employs an electrolytic bath containing 41 to 73 grams of trivalent chromium, to 264 grams of free urea in one liter of aqueous solution. The free urea comprising 2.46 to 6.44 times the weight of trivalent chromium is added to the electrolytic bath. The present inventor has deduced, from the molecular constitution, that the urea molecules existing in a free state in the electrolytic bath should demonstrate a strong buffer action. As a matter of fact, an atmosphere of extremely low hydrogen ion concentration taking place locally in the vicinity of the cathode is considerably weakened by the strong buffer action of free urea in the bath. Consequently, the formation of a pasty basic precipitate around the surface of the cathode, which has hitherto been considered as insurmountable difiiculty, can be prevented almost completely. Thus, the electrodeposition of high quality metal at high efliciency has been made possible. But, when the electrolytic bath contains free urea of more than 6.44 times the weight of trivalent chromium, the urea is liable to deposit along with the metal deposition in the form of a white film, and a good result is diflicult to obtain. On the contrary, when the content of free urea in the electrolytic bath is less than 2.46 times the weight of trivalent chromium, obtaining high quality electrodeposition is difiicult due to the deficiency of a reducing atmosphere in the bath and butter action near the cathode. R. W. Parry and others have already reported on a process (Transactions of The Electrochemical Society, volume 92, 1948, p. 507) wherein an aqueous solution containing hexa-urea-chromic chloride (Cr(urea)s)Cl3 is used. In this case, the electrolytic bath contains always urea of 6.92 times the weight of trivalent chromium. Moreover, it seems that the urea molecules form in the bath complex ions with trivalent chromium. The reason for this is, that it is apparent from a close reading of the aforesaid report by Parry et al. that they used, as described at the end of page 514 and also at the end of page 518 in the discussion of the said report, an aqueous solution of a pure coordination compound (Cr(urea)s)Cl3 that can be isolated. However, no description whatever is found of the fact that urea was used. Moreover, the fact, that, in the present applicants experiment, (Cr(urea)s)2(SO4)3 could be separated with a satisfactory yield from a-AgCl precipitate formed by adding Ag2SO4 to an aqueous solution of (Cr(urea)s) C13 will prove that in the aqueous solution of (Cr(urea)s)Cl3 as in the bath introduced by Parry et al., a major port1on of urea is existent in a form of complex ion It is obvious from the reasons given later in this specification, that in the present invention urea molecules are existent in free state in the electrolytic bath.

The reason therefor is as follows:

(a) According to the experimental study by the present inventor, the state of chromium complex ions in the electrolytic bath of the invention is independent of the presence of urea, and the electric resistance of the bath shows an additive increase with the increase of urea content;

(b) The aforesaid experimental facts with respect to the buffer action of urea or the reducing atmosphere brought about by urea in the electrolytic bath could not be explained if a major portion of urea be assumed to exist in the form of the complex ion, (Cr(urea)s) in the electrolytic bath of the present invention;

It is possible under suitable conditions that a complex ion (Cr(urea) s) would form in the aqueous solution of chromic chloride when urea is added thereto, (refer. E. Wilke Dorfurt und K. Niederer: Zeitschrift fiir anorganische Chemie, Bd. 184, S. 145, 1929, and P. Pfeiffer: Berichte der Deutschen Chemischen Gesellschaft, 36 Iahrg. S. 1926, 1903). However, no record has been found which confirms the formation of the complex ion (Cr(urea)s) in the solution caused by the addition of urea to a green solution of chromic sulfate as in the present invention. In view of the fact that the existence of urea over 6.44 times the weight of trivalent chromium is to be avoided, it is quite obvious that the present invention is entirely different from the process described in the report by P. W. Parry and others. In the preceding example of experiment, the relationship: At+K=Ct as shown in the figure is always true, where the time elapsed after the solution has been prepared is represented by t, the specific conductivity of the solutions (A) and (C) at the end of t by At and Cs respectively, and K is a constant showing electrical conductivity. Considering that A-solution differs only in its constant urea content from C-solution, it is to be seen that an increase of resistivity of the solution caused by the addition of urea is always an additive property. Within the limit indicated in the present invention, it is proved that the urea in the electrolytic bath is existent always in free state.

Further, when the electrolytic bath according to the invention is used, it is possible to obtain a satisfactory electrodeposition, in an electrolysis carried out with an anode of a material such as magnetite or graphite, etc., which shows a low oxidising power against chromium during electrolysis and is insoluble. However, from the industrial point of view the present inventor has found it advisable to employ a lead alloy containing by weight 4 to 12% Sn, 1 to Ag, 0.1 to 2.5% Co and balance Pb as a most optimum anode material. When the said lead alloy is used as anode, various advantages are obtainable as compared with anodes of magnetite or graphite which have usual ly been proposed as means for preventing the anodic oxidation of chromium, and proved extremely inconvenient. The advantages just mentioned above include: The lead alloy is high in electrical conductivity; the terminal voltage is relatively low in the course of electrolysis; forming and fabrication in desirable shapes being easy, it is convenient for electrodeposition on a cathode of relatively intricated shape; preparation from commercial raw material at relatively lower cost.

A lead alloy with 30 to 40% Sn and 0.1 to 0.5% Co has already been proposed by C. G. Fink and others (Transactions of The Electrochemical Society, volume 76, 1939, p. 401) as anode material for electrowinning of manganese. Also, C. G. Fink and others (Transactions of The Electrochemical Society, vol. 46, 1924, p. 349, and ibid.. vol. 49, 1926, p. have introduced as anode material for electrolysis of brine another lead alloy containing Ag as high as 50 to 60%, and described that Ag is extremely useful for protecting an anode from corrosion by chlorine. It has been found by the present inventor that silver is highly effective, even when added to a very small quantity of lead, for lowering the oxygen overvoltage of lead alloy and that it is extremely efi'icient to decrease the anodic oxidation of chromium during the electrolysis of trivalent chromium bath. On the other hand, the electrolytic bath according to the present invention is relatively highly acidic and the electric current is also relatively high. Consequently, it is obvious that the above-mentioned lead alloy relatively low in lead introduced by C. G. Fink and others fora different purpose is not suitable for the purpose of the present invention, because the anode would then be corroded exceedingly. Further, Mantell has introduced (C. L. Mantell: U. S. Patent No. 2,340,400, 1944) a lead alloy containing Co, Sn and Sb as anode material for electrolysis particularly of an aqueous manganese solution, and Lowe et al. (S. P. Lowe et al.: U. S. Patent No. 2,419,722, 1947) has suggested another lead alloy containing Sn, Co and Ag as anode material for use in the electrolysis of an aqueous zinc solution. However, when such lead alloys introduced for other purposes are employed in the present invention, a large amount of hexavalent chromium will accumulate in the electrolytic bath as the electrolysis proceeds, which renders the electrolytic deposition impossible. And when a lead alloy containing a relatively large amount of Sn as in Mantells alloy is used in the present invention, an extraordinary corrosion occurs in the course of electrolysis. When the composition of lead alloy of the invention is compared with those introduced for other purposes such as Mantells (loc. cit.) or that of Lowe et al. (loc. cit.), they are different not only in their uses, but also entirely in their composition ranges as shown in the following:

Lowe at al Mantells the Present Conbmuent Lend Alloy Lead Alloy Invention Percent Percent Percent 4-12 0. 1-2. 6 1-10 none non 0 With this in view, an extraordinarily appropriate lead alloy for anodes has been successfully developed by the present invention through the addition of silver beside tin and cobalt, which, during electrolysis, is hard to corrode due to high content of lead, and besides displays an exceedingly weak oxidising power on chromium, when used as anode. Secondly, though a perfect prevention of the anodic oxidation of chromium during electrolysis is unattainable, even with the use of the lead alloy according to the invention, this causes no practical inconvenience for the reason given below: In the course of the electrolytic operation, divalent chromium in a relatively larger quantity appears on the cathode simultaneously as the electrodeposition of metal commences; in consequence, a small quantity of hexavalent chromium produced by the anodic oxidation is reduced by the divalent chromium; the electrolytic bath according to the present invention, though containing a maximum of 1.5 g. per liter of hexavalent chromium, shows no trouble in practice.

The reason why this is the case according to the opinion of the inventor is explained as follows:

When an aqueous solution containing trivalent chromium and hexavalent chromium is subjected to electrolysis, the cathode is covered with a membrane-like diaphragm consisting of chromic chromate as indicated by Miller et al. (E. Miller: Zeitschrift fiir Elektrochemie, Bd. 32, 8.399, 1926). When the content of hexavalent chromium in the electrolytic bath of the present invention is as low as below 1.5 g./ 1., the membrane of chromic chromate then formed is favorable for the electrolysis. Divalent chromium accumulates within this membrane discharges at the cathode to deposit fine metal. In this case, the free urea molecule develops the aforementioned effect in the diaphragm. In the electrolytic bath of the present invention, when the content of hexavalent chromium is excessively high, a smooth progress of electrolytic deposition seems to be hampered, because the chromic chromate diaphragm covering the cathode during electrolysis is then too thick and too hard.

From the preceding reasons, it will be seen that the accumulation of hexavalent chromium which forms gradually in the electrolytic bath in an extremely small quantity during operation, does not show any obstacle in prac tice. The small quantity of hexavalent chromium can readily be removed by suitable means, for instance, by addition of hydrogen peroxide solution of an appropriate quantity which reduce and remove it.

Further, in the components of the lead alloy proposed in the present invention, if Sn, Ag and Co exceed the above-mentioned range, the anode made thereof is readily subject to corrosion, while, if those components are less than the above limits, the oxidising power of the anode made thereof will be high. In both cases, the alloy is no longer suitable for anode construction.

The facts with respect to the lead alloy employed may be summarized as follows:

(a) The lead alloy used in the present invention is entirely different than those heretofore introduced in its application and also in the range of the individual constituents;

(b) The lead alloys hitherto introduced for other purposes cannot be utilized for the present invention;

(c) Heretofore no reference has been found in regard to an appropriate lead alloy used as an anode material for the electrolysis of an aqueous solution of which the principal constituent is chromic sulfate;

(d) Unless the lead alloy according to the present invention is employed, the electrolytic deposition of the I present invention will not be established industrially.

In the process according to the present invention, the most advantageous electrolytic bath from the point of industrial practice is one that contains chromic sulfate as the principal constituent with free urea and ammonium sulfate as secondary constituents. Under the circumstances, however, a part of the ammonium sulfate can be substituted by either sodium sulfate or potassium sulfate within the limit given below. As is well known, the advantages furnished by those sulfates of ammonium, sodium and potassium added to the electrolytic bath are in the first place that the electrical conductivity of the path is improved thereby, and in the second place, they show buffer characteristics to some extent. The optimum range for the amount of addition of ammonium, sodium and potassium sulfates are as in the following, and whenever the content of those sulfates in the bath may be less or more than those limits, the conductivity will become poor and inconvenient.

When the content of trivalent chromium in a liter of the solution is less than 41 g., the layer of electrolytic deposition formed will be scaly and easy to strip, and if it is more than 73 g., the viscosity and resistivity of the electrolytic bath will become exceedingly high, and inconvenient in both cases.

The details of the process according to the invention are as follows:

Electrolytic bath.-The electrolytic bath used in the present invention contains in a liter: 41 to 73 g. of trivalent chromium; 180 to 264 g. of free urea; 90 to 123 g. of ammonium radical, which can be substituted, to the extent of 55 g. maximum, of either sodium or potassium; sulfate radical stoichiometrically at least equivalent to the content of ammonium radical, sodium, potassium and trivalent chromium.

In the preparation of the electrolytic bath, any one of the following solutions can be chosen: a solution obtained by reducing the hexavalent chromium solution; a solution of chrome-alum dissolved in the water; a solution of commercial chromic sulfate dissolved in the water. And in any of the above solutions, first the theoretical quantities of chemicals necessary are dissolved excepting urea. The solution thus obtained is boiled for several minutes and the theoretical quantity of urea is dissolved therein after being cooled down below 60 C. The procedure of boiling the solution in such a manner as above is not always necessary. This however is convenient for dissolving completely ammonium sulfate, etc., as well as for finding out proper aging time necessary for the complexion state in the electrolytic bath to reach an equilibrium for the electrolytic temperature. The electrolytic bath thus prepared is held at a definite temperature, at which electrolysis is to be conducted, for aging. For the electrolytic bath prepared in the above procedure, each necessary aging time is roughly as follows: over about 500 hours for 25 C., over about 100 hours for 35 C., over about 12 hours for C., and over about 4 hours for C. The solution is used for the electrodeposition after it has been affirmed that the complex-ion state in the bath has reached an equilibrium for the temperature of electrolysis and that the hydrogen ion concentration. shows a pH- value ranging from 1.8 to 3.0. Moreover, the electrolytic bath affords no obstacle in the operation, even when it contains up to 1.5 g. maximum per liter of hexavalent chromium, in accordance with the operation. The electrolytic bath always contains a small quantity of divalent chromium, during or immediately after the electrolysis.

Equipment-An acid-proof, open cell, with a cathode consisting of a conductor to be electrodeposited hanging therein and an anode made of a lead alloy containing by weight 4 to 12% Sn, 1 to.10% Ag, 0.1 to 2.5% Co and balance Pb, is used.

Operati0n.The working temperature of the electrolytic bath is held at a definite temperature between 25 and 55 C.

The cathode current density to be applied ranges appropriately as follows: from 8 to 28 ampere/dm for temperatures of electrolysis of 25 C.; from 14 to 36 ampere/dm. for 35 C.; from 24 to 42 ampere/dm. for 45" C.; from 36 to 46 ampcre/dm. for 55 C. Stirring of the bath during the operation is favorable.

The decomposition of urea which occurs in the bath although extremely low is liable to influence the pH-value of the electrolytic bath. Therefore, the hydrogen ion concentration of the bath is held at pH-values from 1.8 to 3.0 by suitable addition of sulfuric acid. At the same time, urea in an amount equivalent to the addition of sulfuric acid is supplied to the bath.

In some conditions of the electrodeposition, the hexavalent chromium formed due to the anodic oxidation accumulated gradually in the bath. In such case, an appropriate quantity of hydrogen peroxide solution is added in order to reduce the said hexavalent chromium, and thus to assure that the content of hexavalent chromium does not exceed 1.5 grams per liter of the electrolytic bath.

The supply of chromium in the bath is maintained by dissolving the required quantity of chromic acid in the bath, then an appropriate reducing agent, for instance, hydrogen peroxide solution is added in an amount equivalent to the said chromic acid, in order to reduce the hexavalent chromium.

It is also advantageous to add green syrup of chromic sulfate in a suitable quantity when the pH value of the bath is relatively high.

Practical examples illustrating the present invention are given as follows:

Example 1.An aqueous solution is used which contains per liter the following constituents as follows: 196 g. of chromic sulfate; 423 g. of ammonium sulfate; 240 g. of urea. A concentrated aqueous solution of chromic acid is first completely reduced with sulfur dioxide, then the excess sulfur dioxide absorbed is removed with chromic acid added in an equivalent quantity to it, and thus a green aqueous solution containing per liter about 392 g. of chromic sulfate is obtained. A theoretical amount of ammonium sulfate is added to the required quantity of the said solution and stirred thoroughly, while heating above 90 C. A theoretical amount of urea is dissolved therein after it has been cooled down below C. Finally, an appropriate quantity of water is added. Then, an aqueous solution of the above-mentioned chemical composition is obtained. The solution obtained is filtered and held at about 40 C. in an open cell and is subjected to an aging treatment for about 40 hours. Then, the hydrogen ion concentration is adjusted to about 2.1 pH- value. This is used for the electrodeposition.

A lead-lined open cell is employed; the total quantity of the electrolytic bath is 200 liters. As a cathode, a nickel plated copper sheet of 2 dm. per one surface is used, while an anode, which consists of a lead alloy sheet of which the effective area is 8 dm. containing 8% of tin, 4% of silver, 2% of cobalt and 86% of lead, is used by providing them on both sides of the cathode at a distance of 20 cm. from the cathode respectively.

When a cathode current density of 30 amp./dm. is used, the terminal voltage of about 10.5 volt is shown. The agitation of the bath is appropriately conducted by means of the compressed air blown in the bath during the operation.

After the electrolysis has been carried out for 5 minutes in such a manner as above-mentioned, a beautiful, high quality deposit is obtained at the cathode, with the current efi'iciency of about 9% calculated as electrodeposition from trivalent chromium, that is, about 18% calculated as that from hexavalent chromium. Moreover, no evolution of poisonous gas takes place during the operation as in the chromic acid process. Hence, the operation is extremely sanitary. Further, taking the current efliciency of approximately 13% in the case with chromic acid bath into consideration, the current efiiciency for this example may be considered satisfactory as compared with that for the chromic acid bath.

The electrolytic bath of 200 liter has been altered at the end of the operation carried out under a current input of net 540 amp-hour for one day, as follows: The pH value of bath is then 2.4; the decrease of chromium due to the electro-deposition and drug-out is 35 g.; hexavalent chromium accumulated in the bath becomes 50 g. In order to return to the initial condition of the bath before the operation, the following procedure has been conducted. 100 cc. of 96% concentrated sulfuric acid is added to the bath to adjust the pH value of bath, and at the same time 108 g. of urea is also supplemented. Then, 68 g. of chromic acid is dissolved and subsequently 340 cc. of 30% of hydrogen peroxide solution is gradually added in the bath under stirring. Thereby, the hexavalent chromium in the bath could then be reduced completely.

Example 2.An aqueous solution containing the following constituents per liter is used as the electrolytic bath: 196 g. of chromic sulfate; 271 g. of ammonium sul fate; 87 g. of potassium sulfate; 30 g. of sodium sulfate; and 240 g. of urea. In the preparation of above electrolytic bath, first the required amount of chrome-potassium alum is dissolved completely by heating in a small amount of added water. The thus obtained aqueous solution is used. The subsequent procedures in'the preparation are conducted almost similarly as in Example 1, and the aging is also the same as in the preceding case. When the electrolysis is carried out under perfectly the same condition as in Example 1 in except of what has been described above, a satisfactory result nearly the same as Example 1 is obtained. The electrolytic bath, as shown in this example, shows a terminal voltage of about 12 volts, on account of its somewhat low electrical conductivity.

Other procedures in the operation are substantially the same as in Example 1.

What I claim is:

l. A process for the electrodeposition of metallic chromium which comprises causing electrodeposition of metallic chromium from an electrolytic bath consisting of an aqueous solution containing per liter 41 to 73 grams trivalent chromium, 180 to 264 grams free urea, 90 to 123 grams of a substance selected from the group consisting of ammonium, ammonium and potassium, ammonium and sodium, and ammonium, potassium and sodium, the amount of potassium and sodium, when included, not exceeding 55 grams, and sulphate radical in an amount at least stoichiometrically equivalent to the amount of ammonium radical, potassium and sodium when included, and trivalent chromium, said solution having been aged at a. constant electrolytic temperature to cause an equilibrium in the state of the complex ions in the solution at the temperature of electrolysis, said temperature being between 25 and 55 C., With an anode consisting of 4 to 12% tin, 1 to silver, 0.1 to 2.5% cobalt and the balance lead.

2. A process for the electrodeposition of metallic chromium which comprises causing electrodeposition of metallic chromium from an electrolytic bath consisting of an aqueous solution containing per liter 41 to 73 grams trivalent chromium, 180 to 264 grams free urea, to 123 grams of ammonium radical, and sulphate radical in an amount at least stoichiometrically equivalent to the amount of ammonium radical, and trivalent chromium, said solution having been aged at a constant electrolytic temperature to cause an equilibrium in the state of the complex ions in the solution at the temperature of electrolysis, said temperature being between 25 and 55 C., with an anode consisting of 4 to 12% tin, 1 to 10% silver, 0.1 to 2.5% cobalt and the balance lead.

3. A process for the electrodeposition of metallic chromium which comprises causing electrodeposition of metallic chromium from an electrolytic bath consisting of an aqueous solution containing per liter 41 to 73 grams trivalent chromium, to 264 grams free urea, a combined weight of 90 to 123 grams of ammonium radical and sodium, the amount of sodium not exceeding 55 grams and sulphate radical in an amount at least stoichiometrically equivalent to the amount of ammonium radical, sodium and trivalent chromium, said solution having been aged at a constant electrolytic temperature to cause an equilibrium in the state of the complex ions in the solution at the temperature of electrolysis, said temperature being between 25 and 55 C., with an anode consisting of 4 to 12% tin, 1 to 10% silver, 0.1 to 2.5% cobalt and the balance lead.

4. A process for the electrodeposition of metallic chromium which comprises causing electrodeposition of metallic chromium from an electrolytic bath consisting of an aqueous solution containing per liter 41 to 73 grams trivalent chromium, 180 to 264 grams free urea, a combined weight of 90 to 123 grams of ammonium radical and potassium, the amount of potassium not exceeding 55 grams, and sulphate radical in an amount at least stoichiometrically equivalent to the amount of ammonium radical, potassium and trivalent chromium, said solution having been aged at a constant electrolytic temperature to cause an equilibrium in the state of the complex ions in the solution at the temperature of electrolysis, said temperature being between 25 and 55 C., with an anode consisting of 4 to 12% tin, 1 to 10% silver, 0.1 to 2.5% cobalt and the balance lead.

5. A process for the electrodeposition of metallic chromium which comprises causing electrodeposition of metal lic chromium from an electrolytic bath consisting of an aqueous solution containing per liter 41 to 73 grams trivalent chromium, 180 to 264 grams free urea, a combined weight of 90 to 123 grams of ammonium radical, potassium and sodium, the amount of potassium and sodium not exceeding 55 grams and sulphate radical in an amount at least stoichiometrically equivalent to the amount of ammonium radical, potassium and sodium and trivalent chromium, said solution having been aged at a constant electrolytic temperature to cause an equilibrium in the state of the complex ions in the solution at the temperature of electrolysis, said temperature being between 25 and 55 C., with an anode consisting of 4 to 12% tin, 11 t) 10% silver, 0.1 to 2.5% cobalt and the balance 6. An alloy anode for use in the electrodeposition of metallic chromium from an electrolytic bath containing trivalent chromium and sulfate radicals as the principal components, said anode consisting essentially of 4 to 12% by weight of tin, 1 to 10% by weight of silver, 0.1 to 2.5% by weight of cobalt and the balance lead.

References Cited in the file of this patent UNITED STATES PATENTS Mantell Feb. 1, 1944 Lowe et al Apr. 29, 1947 OTHER REFERENCES 

1. A PROCESS FOR THE ELECTRODEPOSITION OF METALLIC CHROMIUM WHICH COMPRISES CAUSING ELECTRODEPOSITION OF METALLIC CHROMIUM FROM A ELECTROYTIC BATH CONSISTING OF AN AQUEOUS SOLUTION CONTAINING PER LITER 41 TO 73 GRAMS TRIVALVENT CHROMIUM, 180 TO 264 GRAMS FREE UREA, 90 TO 123 GRAMS OF A SUBSTANCES SELECTED FROM THE GROUPS CONSISTING OF AMMONIUM, AMMONIUM AND POTASSIUM AMMONIUM AND SODIUM, AND AMMONIUM, POTASSIUM AND SODIUM, THE AMOUNT OF POTASSIUM AND SODIUM WHEN INCLUDED, NOT EXCEEDING 55 GRAMS, AND SULPHAR RDICAL IN AN AMOUNT AT LEAST STOCICHIOMETRICALLY EQUIVALENT TO THE AMOUNT OF AMMONIUM RADICAL, POTASSIUM AND SODIUM WHEN INCLUDED, AND TRIVALENT CHROMIUM, SAID SOLUTION HAVING BEEN AGED AT A CONSTANT ELECTROLYTIC TEMPERATURE TO CAUSE AN EQUILIBRIUM IN THE STATE OF THE COMPLEXTIONS IN THE SOLUTION AT THE TEMPERATURE OF ELECTGROLYSIS, SAID TEMPERATURE BEING BETWEEN 25* AND 55* C., WITH AN ANODE CONSISTING OF 4 TO 12% TIN, 1 TO 10% SILVER, 0.1 TO 2.5% COBALT AND THE BALANCE LEAD. 